Amphibians

Photo by: Sebastian Duda

Amphibians are cold-blooded animals that possess backbones and display
features that lie between those of fish and reptiles. They spend time both
in water and on land. Their larvae (not yet fully developed offspring)
mature in water and breathe through gills, like fish, while adults breathe
air through lungs and skin. Amphibians are in the class Amphibia, which
includes over 3,500 species. They are further divided into three orders:
Anura (frogs and toads), Urodela (salamanders and newts), and Gymnophiona
(caecilians, pronounced sih-SILL-yuhns, which are wormlike in appearance).

History

Amphibians evolved from fish about 400 million years ago, when the amount
of dry land on Earth increased greatly. Certain fish adapted to these
changing conditions by gradually developing limbs to crawl with and lungs
to breathe with. Such organisms, capable of life both in water and on
land, came to be called amphibians, a name that means "double
life." Amphibians were the first vertebrates (animals with
backbones) to live on land. However, they returned to the water to breed.
The largest variety of amphibians occurred about 360 to 230 million years
ago, when the environment was continually alternating between wet and dry
conditions. Many of the species that developed during this period no
longer exist. The groups of amphibians that survived to the present day
can be traced back no further than 200 million years.

Characteristics

Amphibians are cold-blooded animals, meaning they do not have a constant
body temperature but instead take on the temperature of their environment.
They have moist, scaleless skin that absorbs water and oxygen, but that
also makes them vulnerable to dehydration (loss of bodily fluids). Without
moist conditions, their skin dries out and they die. Therefore, amphibians
are most often found near ponds, marshlands, swamps, and other areas where
freshwater is available. Some amphibians become
inactive when conditions are unfavorable for survival. This period of
inactivity is called estivation when it occurs during hot, dry weather and
hibernation when it occurs in response to cold temperatures. Activity
resumes when favorable conditions return.

The thin skin of amphibians contains many glands, among them poison glands
that protect certain species against predators. The poison from the glands
of the brightly colored poison-dart frog is particularly toxic and is used
by South American Indians to coat the tips of their arrows. Some
amphibians protect themselves from enemies by changing color to blend in
with their surroundings.

Life cycle

The life cycle of most amphibians begins in water when the female lays
eggs that are fertilized outside of her body. The eggs then hatch into
larvae, or tadpoles, that breathe through external gills. The larvae grow
flat tails and feed on vegetation. During a process called metamorphosis,
physical changes occur and external gills give way to lungs. The tadpoles
also change from plant-eating animals to meat eaters. Amphibians usually
reach full adulthood at three to four years.

Words to Know

Estivation:
State of inactivity during the hot, dry months of summer.

Gill:
A bodily organ capable of obtaining oxygen from water.

Hibernation:
State of rest or inactivity during the cold winter months.

Invertebrate:
An animal lacking a spinal column.

Larva:
An animal in its early form that does not resemble the parent and must
go through metamorphosis, or change, to reach its adult stage.

Vertebrate:
An animal having a spinal column.

Not all amphibians follow this pattern of reproduction. Some salamanders
live out their entire lives on land, where they give birth to fully formed
live young. Others lay their eggs in moist places on the forest floor,
where they hatch as tiny versions of the adults. Some newts retain
their external gills throughout their lives. The red-spotted newt of
eastern North American spends its juvenile stage on land as the red eft,
returning to water to develop and live as an adult.

Three major groupings

Anurans.
Frogs and toads make up the order Anura, the largest group of living
amphibians, comprising about 3,000 species. Anurans lack tails and have
long hind legs that are well adapted for jumping and swimming. Most
anurans live in areas where there is freshwater, although some are well
adapted to drier habitats. Some common anurans of North America

The life cycle of frogs.
(Reproduced by permission of

The Gale Group

.)

include the bullfrog, spring peeper, American toad, and spadefoot toad.
Frogs and toads differ in that toads have shorter legs and drier skin that
appears warty in comparison to the smooth skin of frogs. Frogs range in
size, the smallest measuring about 1 inch (2.5 centimeters) and the
largest (the West African Goliath frog) measuring more than 1 foot (about
30 centimeters).

Frogs and toads live mainly on a diet of insects and other invertebrates.
The largest frogs and toads also eat small mammals, birds, fish, and other
amphibians.

Urodeles.
The order Urodela contains about 250 species of newts and salamanders.
Urodeles range in size from approximately 4 inches (about 10 centimeters)
to the largest of all amphibians, the giant salamander of Japan, which
grows to more than 5 feet (about 1.5 meters). Urodeles have long tails and
small, underdeveloped legs. They are usually found in or near water and
often reside in moist soil under rocks or logs. Adults usually spend most
of their time on land and have a diet consisting of insects and worms.

Some species of urodeles are aquatic (live in water), including those of
the genus
Siren.
These North American amphibians are shaped like eels, have small forelegs
and no hind legs or pelvis. They breathe through external gills as well as
lungs and burrow in mud at the bottom of marshes.

Gymnophions.
Caecilians of the order Gymnophiona are blind, legless amphibians shaped
like worms. They burrow in moist soil in tropical habitats of Africa and
South America, feeding on soil invertebrates such as worms. There are at
least 160 species of caecilians, ranging in size from 4 inches (about 10
centimeters) to 4.5 feet (about 1 meter) in length, but most are rarely
seen despite their size.

Recent decline

In the last half of the twentieth century, scientists noted the alarming
decline in the numbers of amphibians and amphibians species around the
world. They theorized the decline was due to a number of factors:
pollution of freshwater ecosystems, the destruction of amphibian habitats
by ever-spreading human populations, and, possibly, increased ultraviolet
radiation due to ozone depletion. Amphibians are known as indicator
species, or species whose health is an indicator or sign of the health of
the ecosystem they inhabit. As their numbers decrease, so do the number of
healthy ecosystems around the world, which in turn results in the loss of
many other animal and plant species.

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Nice chart on the life cycle of the frog. My partner and I have been trying to figure out if it's possible that we have huge tadpoles in our pond in mid April. They sure looked like huge tadpoles, but we weren't sure they grew that fast. based on the life cycle chart on this site, I'm guessing that's exactly what they are!

We've had a wildlife pond for 3 years and upto now there has been no frog or toad spawn, although there have been frogs in the pond, 1 last year, now four this year, perhaps a good sign, we think they were all males. Also a couple of frogs were having a good time on the pathway on the 1st March!!! One male with one female. Upto now no spawn. The pond measures approx 2ft deep, 2ft wide, 3-4ft long. With a ledge about six inches down. What are we doing wrong? Is it that frogs and toads are very fussy where they lay their spawn, or are we being impatient We have seen plenty and frogs and toads around the garden, along with lots of newts, which there are lots of young. So newts 1 - frogs and toads O. Please help!!

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American toad, and spadefoot toad. Frogs and toads differ in that toads have shorter legs and drier skin that appears warty in comparison to the smooth skin of frogs. Frogs range in size, the smallest measuring about 1 inch (2.5 centimeters) and the largest (the West African Goliath frog) measuring more than 1 foot (about 30 centimeters).

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